Micropipe-free silicon carbide and related method of manufacture

ABSTRACT

Micropipe-free, single crystal, silicon carbide (SiC) and related methods of manufacture are disclosed. The SiC is grown by placing a source material and seed material on a seed holder in a reaction crucible of the sublimation system, wherein constituent components of the sublimation system including the source material, reaction crucible, and seed holder are substantially free from unintentional impurities. By controlling growth temperature, growth pressure, SiC sublimation flux and composition, and a temperature gradient between the source material and the seed material or the SiC crystal growing on the seed material during the PVT process, micropipe-inducing process instabilities are eliminated and micropipe-free SiC crystal is grown on the seed material.

This application claims the benefit of U.S. Provisional Application No.60/844,360 filed on Sep. 14, 2006.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a micropipe-free silicon carbide,micropipe-free silicon carbide single crystal wafers, and relatedmethods of manufacture.

2. Description of Related Art

Single crystal silicon carbide (SiC) has proven to be a very usefulmaterial in the manufacture of various electronic devices. Due to itsphysical strength and excellent resistance to many chemicals, SiC may beused to fabricate very robust substrates adapted for use in thesemiconductor industry. SiC has excellent electrical properties,including radiation hardness, high breakdown field, a relatively wideband gap, high saturated electron drift velocity, high-temperatureoperation, and absorption and emission of high-energy photons in theblue, violet, and ultraviolet regions of the optical spectrum.

SiC is conventionally produced using various seeded sublimation growthprocesses. Selected seeded sublimation growth processes are disclosed,for example, in U.S. Pat. Nos. 4,912,064, 4,866,005 (U.S. Reissue34,861), and U.S. Pat. No. 5,679,153, the collective subject matter ofwhich is hereby incorporated by reference.

In a typical SiC growth process, a seed material and source material arearranged in a reaction crucible which is then heated to the sublimationtemperature of the source material. By controlled heating of theenvironment surrounding the reaction crucible, a thermal gradient isdeveloped between the sublimating source material and the marginallycooler seed material. By means of the thermal gradient source materialin a vapor phase is transported onto the seed material where itcondenses to grow a bulk crystalline boule. This type of crystallinegrowth process is commonly referred to as physical vapor transport (PVT)process.

In conventional SiC growth processes, the reaction crucible is made ofcarbon (including, for example graphite and/or other carbon materials)and is heated using an inductive or resistive heating technique. Theheating coils and associated insulation are carefully positioned inrelation to the reaction crucible to establish and maintain the desiredthermal gradient. Source material, such as powdered SiC, is commonlyused in conjunction with vertically oriented reaction crucibles. Thepowdered SiC is retained in a lower portion of the reaction crucible andthe seed material is positioned in an upper portion of the reactioncrucible during the PVT process.

The unique properties of SiC substrates enable the design andfabrication of an array of high power and/or high frequencysemiconductor devices. Continuous development over the past decade haslead to a level of maturity in the fabrication of SiC wafers that allowssuch semiconductor devices to be manufactured at commercially acceptableprice points. However, certain materials-related problems continue toimpede the broader use of SiC wafers as the substrate of choice in manycommercial applications. These materials-related problems are largelythe result of certain structural defects in the material composition ofconventionally manufactured SiC, such as micropipes, dislocations (e.g.,threading, edge, basal plane, and/or screw dislocations), hexagonalvoids, stacking faults, etc. Each of these structural defects is causedby one or more discontinuities in the material lattice structure of theconstituent SiC. Such structural defects are detrimental to fabricationand proper operation of semiconductor devices subsequently formed on theSiC substrate. Device yield and reliability suffer accordingly.

The nature and description of structural defects in SiC are wellunderstood, and although the density of such defects has been reducedover time, relatively high defect concentrations still appear and haveproven difficult to eliminate. (See, for example, Nakamura et al.,“Ultrahigh quality silicon carbide single crystals,” Nature, Vol. 430 atpage 1009, (Aug. 26, 2004)).

Micropipes are common structural defects in SiC that develop orpropagate during seeded sublimation growth processes. A micropipe is ahollow core, super-screw dislocation with its Burgers vector typicallylying along the c-axis. A number of causes have been attributed to thegeneration of micropipes in SiC. These include inclusions of excessmaterials such as silicon or carbon inclusions, extrinsic impuritiessuch as metal deposits, boundary defects, and the movement or slippageof partial dislocations. (See, e.g., Powell et al., Growth of lowmicropipe density SiC wafers,” Materials Science Forum, Vols. 338-40,pp. 437-40 (2000)).

Other evidence suggests that micropipes are associated with hexagonalvoids. Hexagonal voids are flat, hexagonal platelet-shaped cavities inthe crystal that often have hollow tubes trailing beneath them. (See,e.g., Kuhr et al., “Hexagonal voids and the formation of micropipesduring SiC sublimation growth,” Journal of Applied Physics, Vol. 89, No.8 at page 4625 (April 2001)).

With continuing efforts to commercialize SiC substrates and devicesformed on SiC substrates, an increased emphasis has been placed on thequality of the constituent substrate material. For example, theso-called repeated a-face (RAF) growth approach has demonstratedmicropipe-free SiC samples. However, this process can not be adapted foruse in commercial applications due to its long process cycle time,complicated processing, difficulties in wafer diameter expansion andcorresponding expense. Thus, while the industry has been able tosteadily decrease the density of micropipes in commercial SiC substratesover time, there is a need for a SiC wafer growth process that iscommercially viable and that provides single crystal SiC which isentirely free of micropipes.

SUMMARY OF THE INVENTION

Embodiments of the invention are directed to a micropipe-free siliconcarbide (SiC), micropipe-free single crystal SiC wafers and substrates,and related methods of manufacture.

In one embodiment, the invention provides a method of growing asingle-crystal of silicon carbide (SiC crystal) in the nominal c-axisgrowth direction using a physical vapor transport (PVT) process in asublimation system, wherein the crystal is completely free of micropipedefects, the method comprising; attaching a seed material to a seedholder and forming a uniform thermal contact between the seed materialand seed holder, placing a source material and the seed materialattached to the seed holder in a reaction crucible, wherein constituentcomponents of the sublimation system including at least the sourcematerial, the seed holder, and the reaction crucible are substantiallyfree from unintentional impurities, and controlling growth temperature,growth pressure, SiC sublimation flux and composition, and a temperaturegradient between the source material and the seed material or the SiCcrystal growing on the seed material during the PVT process to eliminatemicropipe-inducing process instabilities and grow the micropipe-free SiCcrystal on the seed material.

In another embodiment, the invention provides a micropipe-free siliconcarbide (SiC) wafer sliced from a SiC crystal grown in the nominalc-axis direction by a process of placing a source material and a seedmaterial in a reaction crucible of a sublimation system, whereinconstituent components of the sublimation system including at least thesource material and the reaction crucible are substantially free fromunintentional impurities, and controlling growth temperature, growthpressure, SiC sublimation flux and composition, and a temperaturegradient between the source material and the seed material or a SiCcrystal growing on the seed material during a physical vapor transport(PVT) process to eliminate micropipe-inducing process instabilities.

In another embodiment, the invention provides a semiconductor wafercomprising; a bulk single crystal silicon carbide (SiC) substrate slicedfrom a crystal grown in the nominal c-axis direction and having amicropipe density of zero, the SiC substrate comprising opposing firstand second surfaces, an epitaxial layer formed on at least the firstsurface of the SiC substrate and comprising a concentration of dopantatoms defining a conductivity for the epitaxial layer, and asemiconductor device comprising source/drain regions formed in theepitaxial layer and defining a channel region in the epitaxial layer.

In another embodiment, the invention provides a semiconductor wafercomprising; a bulk single crystal silicon carbide (SiC) substrate slicedfrom a crystal grown in the nominal c-axis direction and having amicropipe density of zero, the SiC substrate comprising opposing firstand second surfaces, an epitaxial layer formed on at least the firstsurface of the SiC substrate and comprising a concentration of dopantatoms defining a conductivity for the epitaxial layer, and asemiconductor device formed at least in part in the epitaxial layer.

In another embodiment, the invention provides a method of growing asingle-crystal of Group III-nitride (Group III-nitride crystal) in anominal c-axis growth direction using a physical vapor transport (PVT)process in a sublimation system, wherein the Group III-nitride crystalis completely free of micropipe defects, the method comprising;attaching a seed material to a seed holder and forming a uniform thermalcontact between the seed material and seed holder, placing a sourcematerial and the seed material attached to the seed holder in a reactioncrucible, wherein constituent components of the sublimation systemincluding at least the source material, the seed holder, and thereaction crucible are substantially free from unintentional impurities,and controlling growth temperature, growth pressure, Group III-nitridesublimation flux and composition, and a temperature gradient between thesource material and the seed material or the Group III-nitride crystalgrowing on the seed material during the PVT process to eliminatemicropipe-inducing process instabilities and grow the micropipe-freeGroup III-nitride crystal on the seed material.

In another embodiment, the invention provides a semiconductor wafercomprising; a bulk single crystal silicon carbide (SiC) substrate slicedfrom a crystal having a minimum diameter of 3 inches and having amicropipe density of zero, the SiC substrate comprising opposing firstand second surfaces, and an epitaxial layer formed on at least the firstsurface of the SiC substrate and comprising a concentration of dopantatoms defining a conductivity for the epitaxial layer.

DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic cross-sectional view of a seeded sublimationsystem in accordance with an embodiment of the present invention;

FIGS. 2( a) and 2(b) are photographic images respectively showing anetched surface of a 6H—SiC crystal (initial stage of growth), anddistribution of polytype inclusions in 6H—SiC boule (bulk growth stage);

FIG. 3 is an image showing micropipe formation at 6H-15R polytypeboundaries; and

FIGS. 4( a)-4(c) are photographs respectively showing micropipe-free4H—SiC substrates including an optical image of a 2-inch diameter wafer,a cross-polarized image of a 2-inch diameter wafer, and across-polarized image of a 3-inch diameter wafer according to variousembodiments of the invention.

FIG. 5 is a photograph showing a 100 mm diameter micropipe-free 4H—SiCwafer and fabricated in accordance with an embodiment of the invention.

DESCRIPTION OF EMBODIMENTS

Embodiments of the invention will now be described in relation to theaccompanying drawings. However, the invention is not limited to only theembodiments described herein. Rather, the selected embodiments arepresented as teaching examples.

For purposes of clarity, certain embodiments of the invention aredescribed in the context of known processes and related equipment. Thoseof ordinary skill in the art will recognize that the scope of theinvention are not limited to only these processes and associatedequipment. Rather, as SiC growth processes and associated equipmentchange and evolve over the succeeding years, the teachings of thepresent invention will find ready application within these new contexts.

As is conventionally understood, a SiC boule may be grown using a seededsublimation growth process, such as PVT, using a seeded sublimationsystem. Alternately, seeded sublimation systems may include hightemperature CVD (HT-CVD) systems and halide CVD (H-CVD) systems. Aresulting SiC boule may then be sliced using conventional techniquesinto wafers, and the individual wafers may then be used as seed materialfor a seeded sublimation growth process, or as substrates upon which avariety of semiconductor devices may be formed.

As previously noted, the general aspects of seeded sublimation growthprocesses for SiC are well established. Furthermore, those skilled inthe field of crystal growth and particularly those skilled in the fieldof SiC growth and related systems will recognize that the details of agiven technique can and will vary, usually purposefully, depending onmany relevant circumstances and processing conditions. Accordingly, thedescriptions given herein are most appropriately given in a general andschematic sense with the recognition that those persons of skill in theart will be able to implement and use various embodiments of theinvention based on the provided disclosure without undueexperimentation.

FIG. 1 is a cross sectional schematic diagram of a sublimation system 12adapted for use in a seeded sublimation growth process of the typecontemplated by certain embodiments of the invention. As in mostconventional systems, sublimation system 12 includes a carbon reactioncrucible (also referred to as a susceptor or growth cell) 14 and aplurality of induction coils 16 adapted to heat reaction crucible 14when electrical current is applied. Alternatively, a resistive heatingapproach may be applied to the heating of reaction crucible 14. Usingany competent heating mechanism and approach, the temperature within afurnace housing sublimation system 12 may be conventionallycontrollable.

The furnace housing sublimation system 12 will also typically includeone or more gas inlet and gas outlet ports and associated equipmentallowing the controlled introduction and evacuation of gas from anenvironment surrounding reaction crucible 14. The introduction andevacuation of various gases to/from the environment surrounding reactioncrucible 14 may be conventionally accomplished using a variety ofinlets/outlets, pipes, valves, pumps, gas sources, and controllers.

It will be further understood by those skilled in the art thatsublimation system 12 may further incorporate in certain embodiments awater-cooled quartz vessel. Such further elements are, however, lessrelevant to embodiments of the invention and are omitted as being knownin the art.

Additionally, those skilled in this art will recognize that SiCsublimation systems of the type described herein are commerciallyavailable in various standard configurations. Alternately, sublimationsystems may be designed and implemented in custom configurations, wherenecessary or appropriate. Accordingly, the method embodiments describedherein are not limited to a particular subset of sublimation systems, orany particular system configuration. Rather, many different types andconfigurations of sublimation systems may be used to grow micropipe-freeSiC material in accordance with an embodiment of the invention.

Returning to FIG. 1, reaction crucible 14 is surrounded by insulationmaterial 18. The composition, size, and placement of insulation material18 will vary with individual sublimation systems 12 in order to defineand/or maintain desired thermal gradients (both axially and radially) inrelation to reaction crucible 14. For purposes of clarity, the term,“thermal gradient,” will be used herein to describe one or more thermalgradient(s) associated with reaction crucible 14. Those skilled in theart recognize that “the thermal gradient” established in embodiments ofthe invention will contain (or may be further characterized as having)axial and radial gradients, or may be characterized by a plurality ofisotherms.

Prior to establishment of the thermal gradient, reaction crucible 14 isloaded with one or more source materials. Conventionally availablereaction crucibles include one or more portions, as least one of whichis capable of holding source material, such as powdered SiC 20. Powderedsource material(s) are most commonly, but not exclusively, used inseeded sublimation growth processes. As illustrated in FIG. 1, sourcematerial 20 is held in a lower portion of reaction crucible 14, as iscommon for one type of reaction crucible 14. Other competent reactioncrucibles distribute source material in a vertical or cylindricalarrangement in which the source material surrounds a considerableportion of the interior of reaction crucible 14. Here again, reactioncrucible 14 may be implemented in a number of different shapes, and mayhold one or more source materials accordingly. Thus, while embodimentsof the invention may use conventional reaction crucible designs, thescope of the present invention is not limited to such designs, but willfind application in different sublimation systems using many differenttypes of reaction crucibles.

However, returning again to FIG. 1, a seed material 22 is placed aboveor in an upper portion of reaction crucible 14. Seed material 22 maytake the form of a mono-crystalline SiC seed wafer having a diameter ofabout 2 inches, 3 inches (75 mm) or 4 inches (100 mm). A SiC singlecrystal boule 26 will be grown from seed material 22 during the seededsublimation growth process.

In the embodiment illustrated in FIG. 1, a seed holder 24 is used tohold seed material 22. Seed holder 24 is securely attached to reactioncrucible 14 in an appropriate fashion using conventional techniques. Forexample, in the orientation illustrated in FIG. 1, seed holder 24 isattached to an uppermost portion of reaction crucible 14 to hold seedmaterial 22 in a desired position. In one embodiment, seed holder 24 isfabricated from carbon.

In certain embodiments of the invention, one or more type of dopantatoms may be intentionally introduced into sublimation system 12 duringor before the seeded sublimation process. For example, one or moredopant gases may be introduced into the seeded sublimation environmentand thereby incorporated into the growing SiC crystalline boule. Dopantsmay be selected for their acceptor or donor capabilities in accordancewith the conductivity properties desired for the resulting SiC boule.For certain semiconductor devices, donor dopants produce n-typeconductivity and acceptor dopants produce p-type conductivity. Somecommonly incorporated n-type dopants include N, P, As, Sb, and/or Ti.Some commonly incorporated p-type dopants include B, Al, Ga, Be, Er,and/or Sc.

In a typical sublimation growth process according to an embodiment ofthe invention, an electrical current having a defined frequency to whichthe material (e.g., carbon) forming reaction crucible 14 will respond ispassed through induction coils 16 to heat reaction crucible 14. Theamount and placement of insulation material 18 are selected to create athermal gradient between a source material 20 and a seed material 22.Reaction crucible 14 is heated with source material 20 to a sublimationtemperature above about 2000° C., and typically ranging from about 2100°C. to 2500° C. In this manner, a thermal gradient is established suchthat the temperature of seed material 22 and the SiC crystalline boule26 growing on the seed material 22 remains slightly below thetemperature of source material 20. In this manner, certain vaporizedspecies generated from the sublimating SiC source (e.g., Si, Si₂C and/orSiC₂) are thermodynamically transported first to seed material 22 andthereafter to the growing SiC crystalline boule 26 (or “the SiCcrystal”).

Once the SiC crystal 26 has reached a desired size, the crystal growthprocess is terminated by reducing the temperature of sublimation system12 below about 1900° C. and/or raising the pressure of the environmentsurrounding reaction crucible 14 above about 400 Torr.

With the foregoing exemplary system and process in mind and in view ofrecent studies concerning the control of nucleation and propagation ofmicropipe defects in SiC crystals grown using PVT processes, it has beendetermined that during bulk growth of the SiC boule, foreign polytypenucleation, such as 3C-polytype, occurs at the initial stages of growth(nucleation period) and/or during subsequent growth in the presence offacets. These observations suggest that polytype instability duringcrystal growth adversely impacts micropipe density. Based on thisconcept, embodiments of the invention as hereinafter described focus onthe optimization of growth conditions for nucleation and growth stages.These optimized growth conditions implemented in a PVT growthenvironment provide a growth process that realizes highly effectivepolytype control. Under these growth conditions, micropipes induced onthe seed material and/or formed during growth are practically, if notcompletely, eliminated. In fact, micropipe-free 2-inch, 3-inch, and 100mm diameter 4H—SiC crystals, as well as 2-inch and 3-inch 6H—SiCcrystals have been demonstrated using various embodiments of theinvention.

During SiC bulk growth using a seeded sublimation process as describedabove, foreign polytype nucleation, such as 3C-polytype, typicallyoccurs at the initial stages (nucleation period) and may even continueto nucleate during subsequent growth stages in the presence of the(0001) facet at the growth front, as shown in FIGS. 2( a) and 2(b).Similarly, 15R-polytype may nucleate on 3C-polytype inclusions both in4H— and 6H—SiC boules. Instability in process parameters, such aspressure and temperature during the initial stage of growth, may causepolytype inclusion.

The presence of facet boundaries, which can form at different stages ofthe bulk growth process, increases the probability of polytypenucleation due to the reduced surface energy along these interfaces.Step-bunching and increased dislocation densities generally form alongthe [11 ² 0] direction (and its family). In the vicinity of facetboundaries, the density of such defects increases further. Additionally,the concentration of unintended impurities tends to increase in theseareas, causing stress generation and lattice mismatch. The presence offacet boundaries and polytype inclusions, extending from atomic levelsto macroscopic scales, leads to increased defect levels includingmicropipe formation in SiC crystals. It should also be noted that theconcentration and/or the composition of SiC source flux plays a criticalrole in forming defects discussed above. As such, the growth processparameters are carefully examined to find suitable conditions so thatpolytype inclusions and defect formation around such facet boundariesare reduced and/or prevented.

On the etched surface of the 6H—SiC crystal representing the initialstage of the growth process as shown in FIG. 2( a), anisotropy of thegrowth steps distribution from the center of the crystal to the crystaledge produces a discernable contrast. Along the [1 ¹ 00]crystallographic direction, surface morphology suggests the presence ofa step-flow growth mechanism. Along this [11 ² 0] direction, stepbunching formation is observed, likely indicating increased growth ratein this direction. It is expected that differences in growth rate along[1 ¹ 00] and [11 ² 0] crystallographic directions may result in stepdeformation and generation of a large number of kinks from whichdislocations are formed. Such conditions promote the formation ofpolytype inclusions, which thus result in formation of micropipes. FIG.2( b) shows the 15R polytype inclusion in a 6H crystal occurring in the[11 ² 0] direction and its family. This is confirmed byphotoluminescence and UV absorption characteristics.

A cross-section of a 6H—SiC boule (cut along the c-axis direction)containing 15R-polytype inclusions is shown in FIG. 3. Formation ofmicropipe clusters is shown on the 6H/15R boundaries.

As an example, and consistent with the above noted studies, it has beenobserved that: (1) step bunching occurs at the facet border in the [11 ²0] direction; (2) the facet border is the preferred area for newpolytype nucleation which generally propagates in the [11 ² 0]direction(s); and (3) micropipe generation starts from polytypeinclusions which are nucleated at polytype boundaries.

In a method of growing a micropipe-free, single crystal boule of SiCaccording to an embodiment of this application, the SiC boule is grownnominally in the c-axis direction, where the nominal c-axis direction isdefined as being within the range of zero to 10 degrees from the c-axis[0001] direction. Initially, unintentional impurities within the system(as compared with dopants intentionally introduced into the sublimationsystem) are targeted for elimination as a first possible source ofmicropipe nucleation. In particular, unintentional impurities on theseed or growth front may start to nucleate a micropipe and/or micropipeclusters that may propagate throughout the SiC boule.

For example, unintentional impurities within the source material (e.g.,the powdered SiC source material 20 in the sublimation system as shownin FIG. 1) may be eliminated by providing a very high quality SiC sourcematerial having as low an impurity content as possible. Common sourcematerial impurities include, as examples, iron, nickel and/or chromium.Where very high quality SiC source material of verified purity may beobtained, its incorporation within an embodiment of the invention iswarranted. However, one or more high quality source materials (aspotentially provided in solid, powder, liquid, and/or gaseous form(s))may be employed as SiC source material in a sublimation or evaporationsystem consistent with certain embodiments of the invention where suchsource material(s) contain a total concentration of less than 5 partsper million by weight (ppmwt) of metallic impurities, and morepreferably less than 2 ppmwt of metallic impurities. In this context,metallic impurities include at least metals such as Ti, V, Cr, Mn, Fe,Co, Ni, Cu, Zn, Zr, Mo, Pd, Ta, and/or W. Indeed, certain embodimentsfor the invention provide that the SiC source material(s) being usedcontain a total concentration of less than 1 ppmwt of Ti, V, Cr, Mn, Fe,Co, Ni, Cu, Zn, Zr, Mo, Pd, Ta, and/or W.

In addition to the provision of very high quality source material(s),certain embodiments of the invention demand that the reaction crucibleand seed material holder be fabricated from high quality, very purecarbon having as low a metal content as possible. In this regard, thereduction of metallic impurities in the materials forming the source,reaction crucible, and seed holder may be confirmed using glow dischargemass spectroscopy (GDMS) and secondary ion mass spectroscopy (SIMS)analysis techniques.

Following the foregoing efforts to eliminate unintentional impurities asa source of micropipe generation, polytype inclusions (as previouslydescribed) are targeted as the remaining source for micropipegeneration. By precise and tight control of vapor phase composition, thediffusion rate of the vapor species, growth temperature, pressure duringbulk growth, source flux, ambient gases and growth zone dimensions suchpolytype inclusions may be eliminated. More particularly, instabilitiesduring crystal growth, such as temperature spikes, pressure deviationsor any changes in growth conditions, may cause polytype inclusions.Tight control of these processing parameters to maintain stable growthconditions eliminates polytype inclusions, thus preventing micropipeformation.

It should be understood as noted above, those familiar with the growthof crystals, particularly in difficult material systems such as siliconcarbide, will recognize that the details of the given technique can varydepending on relevant circumstances. Typically, growth pressure duringan applied PVT process will range from about 0.1 to 400 Torr, and moretypically between 0.1 and 100 Torr. As noted above, the processtemperature will range from about 2000° C. to 2500° C. These conditionsmay vary due to differences in the sublimation system being used andvariations in the seeded sublimation growth process being run.

However, regardless of system and process variables, embodiments of theinvention demand that the desired growth pressure and processtemperature be strictly controlled without significant transients, inorder to eliminate the occurrence of polytype inclusions and therebyprevent the formation of micropipes.

In accordance with various embodiments of the invention, 2-inch, 3-inch,and 100 mm diameter wafers of conductive 4H—SiC, and 2-inch and 3-inchsemi-insulating 6H—SiC single crystals have been produced as shown inFIGS. 4( a), 4(b), 4(c) and FIG. 5. It should be noted that thesecrystals, formed in accordance with an embodiment of the invention, werecompletely free of micropipes. These early examples of material obtainedby embodiments of the invention are not limiting. Indeed, the presentinvention is scalable to the fabrication of 125 mm, 150 mm, and greaterdiameter crystals of various materials having different conductivitytypes.

Using the foregoing sublimation system, or an equivalent, certainembodiments of invention provide various methods of growingsingle-crystal SiC that is completely free of micropipe defects. Thesemethods employ PVT to grow the SiC and generally provide for crystalgrowth along the nominal c-axis direction. However, embodiments of theinvention are applicable to SiC crystal growth in a nominal directionwithin a range of from zero to 10 degrees from the c-axis direction. Inone more specific embodiment, the growth direction is within 4 degreesof the c-axis direction with the tilt toward the [11 ² 0] or the [1 ¹00] direction.

As noted above, crystal growth proceeds from a high quality seed. Incertain embodiments of the invention, the seed may be implemented inwafer form from a material characterized by a micropipe density of lessthan 2 cm⁻², and more preferably by a micropipe density of less than 1cm⁻².

Alternately, an acceptable SiC seed wafer may be characterized by onehaving a uniform x-ray diffraction exhibiting a full width at halfmaximum of less than 50 arcsec, and more preferably less than 30 arcsec.Alternately, an acceptable SiC seed wafer is one characterized by noextraneous polytype inclusions.

The attachment of the seed material (i.e., a seed wafer) to acorresponding seed holder within a sublimation system should be made bymeans of a uniform thermal contact. Various conventionally understoodtechniques may be used to implement a uniform thermal contact. Forexample, the seed material may be placed in direct physical contact withthe seed holder, or an adhesive may be used to fix the seed material tothe seed holder, so as to ensure that conductive and/or radiative heattransfer is uniform over substantially the entire area between the seedand the seed holder. Alternately, a wafer holder comprising a controlledgap structure may be used to define and maintain a desired separationgap between the seed material and the seed holder. It will also beunderstood by those skilled in the art that the use of a controlled gapstructure requires a protective backside surface coating on the seedmaterial (i.e., on the surface opposite to the growth surface) so thatthe seed material will not inadvertently sublimate during the growthprocess. In various embodiments of the invention, a controlled gapstructure may be used to form a separation distance between the seedmaterial and seed holder of 10 μm or less, 5 μm or less, 2 μm or less,and where practically possible less than 1 μm.

With an appropriate seed material selected (and its compositional purityverified), and with the seed material properly attached to a seed holderin a reaction crucible environment substantially free from metallicimpurities, the elimination of micropipe-inducing process instabilitiesbecomes the primary focus of various embodiments of the invention. Bycarefully controlling process conditions using known automated,semi-automated, and human directed control techniques the temperature,pressure, temperature gradient, and resulting SiC sublimation flux maybe precisely defined and maintained over a desired process period.

In certain embodiments of the invention, growth pressure is controlledin a range of about 300 to 0.1 torr, and more preferably in a range ofabout 50 to 0.1 torr. The process temperature is controlled in a rangeof about 2000 to 2500° C. The thermal gradient between the growingcrystal and the source material is controlled in a range of about 50 to150° C./cm. A sublimating SiC species flux during the process period maybe controlled by a ramped increase in the growth temperature in therange of 0.3 to 10° C./hr.

It has been clearly demonstrated by certain CVD and liquid phase SiCepitaxial studies, (see, for example, J. Sumakeris et al., 5th EuropeanConference on Silicon Carbide and Related Materials (2004) and U.S. Pat.No. 5,679,153), that micropipes in single crystal SiC substrates can bereduced and/or eliminated by over-grown SiC epitaxy layers. This resultmay be achieved by controlling the input quantities of Si and Creactants, and thereby controlling the Si/C ratio during epitaxialgrowth. Such overgrowth under slightly silicon-rich conditionsdissociates the large Burgers vector micropipes in the single crystalSiC substrate into a number of threading screw dislocations havingsmaller Burgers vectors that do not form open pipes structures in theresulting epitaxial layer.

Thus, in certain embodiments of the invention, it has been found thatthis particular micropipe reduction (elimination) mechanism may beextended to a sublimation growth method for bulk, as opposed toepitaxial, SiC crystals. This inventive extension proposes that at thebeginning of the overall growth process (i.e., the growth nucleationphase), micropipes in seed material may be overgrown by; using a lowmicropipe density, but not necessarily micropipe-free seed material, andcontrolling the growth nucleation temperature, such that a modified Si/Cratio is induced by a low “start temperature” which promotes theclosing-off of any micropipes existing in the seed material during theinitial stages of bulk growth. In one more particular embodiment of theinvention, this start temperature ranges between about 2000 to 2200° C.

As is conventionally understood, a sublimation process consistent withan embodiment of the invention may include the steps of first evacuatingthe environment around the reaction crucible to remove ambient air,gaseous impurities and extraneous solid particulates. Then, the reactioncrucible is placed under pressure using one or more inert gas(es). Then,the sublimation system heats the reaction crucible environment to atemperature enabling SiC crystal growth via PVT. Once this temperatureis reached, the pressure within the sublimation system is reduced to apoint sufficient to initiate SiC crystal growth.

Micropipe-free SiC crystals of various polytype may be grown includingat least 3C, 4H, 6H, and 15R. Wafers cut from such SiC crystals may besubsequently used in the fabrication of various substrates. For example,using known techniques, high quality semiconductor wafers may befabricated that include homo-epitaxial layers, such as SiC, as well ashetero-epitaxial layers, such as Group III-nitrides, on at least onesurface thereof. The Group III-nitride layer may be, for example, GaN,AlGaN, AlN, AlInGaN, InN, and/or AlInN.

In this manner, semiconductor wafers may be fabricated that include abulk single crystal SiC substrate having a diameter of at least 2inches, 3 inches, 4 inches (100 mm) or larger and having a micropipedensity of zero. The SiC substrate will include at least one andpossibly two primary (and opposing) surfaces. As is conventionallyunderstood, a plurality of active and/or passive devices may befabricated on the SiC substrate.

In one embodiment, prior to the formation of the plurality of devices,an epitaxial layer may be formed on the primary surface of thesubstrate. This epitaxial layer may include a concentration of dopantatoms, sufficient to define a desired conductivity for the epitaxiallayer. Source/drain regions may then be formed in the epitaxial layer todefine channel regions for the devices. Thereafter, conventionalsemiconductor fabrication processes may be sequentially applied to thesurface of the SiC wafer to form desired semiconductor devices, such astransistor(s) having conventional gate structure(s) (e.g., oxide ormetal gate structures) formed over defined channel region(s). Selectedexamples of such transistors include; metal oxide semiconductorfield-effect transistors, junction field-effect transistors,hetero-field-effect transistors, and metal semiconductor field-effecttransistors.

Alternatively, in other embodiments of the invention, prior to theformation of semiconductor device(s), an epitaxial layer is formed on atleast one primary surface of the substrate. The epitaxial layer mayinclude selected concentration(s) of dopant atoms sufficient to define adesired conductivity for the epitaxial layer. Thus formed, the epitaxiallayer may be used to fabricate one or more Schottky barrier diode(s),junction barrier Schottky diode(s), PiN diode(s), thyristor(s), and/orbipolar junction transistors on the epitaxial layer.

In view of the foregoing description, micropipe-free substrates may beused to broaden the application of SiC material in the fabrication ofsemiconductor devices and electronic devices. It should be noted thatthe foregoing embodiments are not intended to be an exhaustiverecitation of the scope of the subject invention. Rather, those skilledin the art will recognize that various modifications and adaptations tothe foregoing may be made without departing from the scope of theinvention as defined by the appended claims.

For example, in one such modification, any one of the foregoing methodembodiments may be adapted to grow a Group III-nitride on seed material.That is, a single-crystal of Group III-nitride (Group III-nitridecrystal) having a nominally c-axis growth direction may be grown using aphysical vapor transport (PVT) process in a sublimation system. Such aGroup III-nitride crystal will be completely free of micropipe defects.One method for growing a Group III-nitride crystal includes; attaching aseed material to a seed holder using a uniform thermal contact, placinga source material and the seed material attached to the seed holder in areaction crucible associated with the sublimation system, whereinconstituent components of the sublimation system including at least thesource material, the seed holder, and the reaction crucible aresubstantially free from unintentional impurities, and controlling growthtemperature, growth pressure, Group III-nitride sublimation flux andcomposition, and a temperature gradient between the source material andthe seed material or the Group III-nitride crystal growing on the seedmaterial during the PVT process to eliminate micropipe-inducing processinstabilities and grow the micropipe-free Group III-nitride crystal onthe seed material.

What is claimed is:
 1. A micropipe-free silicon carbide (SiC) waferhaving a minimum diameter selected from a group of diameters consistingof at least 2 inches, at least 3 inches, and at least 100 mm, slicedfrom a SiC crystal grown in the nominal c-axis direction by a process ofplacing a source material and a seed material in a reaction crucible ofa sublimation system, wherein constituent components of the sublimationsystem including at least the source material and the reaction crucibleare substantially free from unintentional impurities, and controllinggrowth temperature, growth pressure, SiC sublimation flux andcomposition, and a temperature gradient between the source material andthe seed material or a SiC crystal growing on the seed material during aphysical vapor transport (PVT) process to eliminate micropipe-inducingprocess instabilities wherein the micropipe-free silicon carbide (SiC)wafer has a micropipe density of zero.
 2. A micropipe-free siliconcarbide (SiC) wafer having a minimum diameter of 100 mm to 125 mm slicedfrom a SiC crystal grown in the nominal c-axis direction by a process ofplacing a source material and a seed material in a reaction crucible ofa sublimation system, wherein constituent components of the sublimationsystem including at least the source material and the reaction crucibleare substantially free from unintentional impurities, and controllinggrowth temperature, growth pressure, SiC sublimation flux andcomposition, and a temperature gradient between the source material andthe seed material or a SiC crystal growing on the seed material during aphysical vapor transport (PVT) process to eliminate micropipe-inducingprocess instabilities wherein the micropipe-free silicon carbide (SiC)wafer has a micropipe density of zero.
 3. The micropipe-free SiC waferof claim 2, comprising a homo-epitaxial SiC layer formed on a principalsurface of the wafer.
 4. The micropipe-free SiC wafer of claim 2,comprising a hetero-epitaxial Group III-nitride layer formed on aprincipal surface of the wafer.
 5. The micropipe-free SiC wafer of claim4, wherein the Group III-nitride layer comprises at least one selectedfrom a group consisting of GaN, AlGaN, AN, AlInGaN, InN, and AlInN.
 6. Amicropipe-free silicon carbide (SiC) wafer having a minimum diameter ofat least 100 mm to 150 mm sliced from a SiC crystal grown in the nominalc-axis direction by a process of placing a source material and a seedmaterial in a reaction crucible of a sublimation system, whereinconstituent components of the sublimation system including at least thesource material and the reaction crucible are substantially free fromunintentional impurities, and controlling growth temperature, growthpressure, SiC sublimation flux and composition, and a temperaturegradient between the source material and the seed material or a SiCcrystal growing on the seed material during a physical vapor transport(PVT) process to eliminate micropipe-inducing process instabilitieswherein the micropipe-free silicon carbide (SiC) wafer has a micropipedensity of zero.
 7. The micropipe-free SiC wafer of claim 6 comprising ahomo-epitaxial layer formed on a principal surface of the wafer.
 8. Themicropipe-free SiC wafer of claim 7, wherein the homo-epitaxial layer isSiC.
 9. The micropipe-free SiC wafer of claim 6 comprising ahetero-epitaxial layer formed on a principal surface of the wafer. 10.The micropipe-free SiC wafer of claim 9, wherein the hetero-epitaxiallayer is a Group III-nitride layer.
 11. The micropipe-free SiC wafer ofclaim 10, wherein the Group III-nitride layer comprises at least oneselected from a group consisting of GaN, AlGaN, AlN, AlInGaN, InN, andAlInN.
 12. The micropipe-free SiC wafer of claim 6, having a polytypeselected from a group of polytypes consisting of 3C, 4H, 6H, and 15R.13. The micropipe-free SiC wafer of claim 6 comprising a homo-epitaxialSiC layer formed on a principal surface of the wafer.
 14. Themicropipe-free SiC wafer of claim 6 comprising a hetero-epitaxial GroupIII-nitride layer formed on a principal surface of the wafer.
 15. Themicropipe-free SiC wafer of claim 14, wherein the Group III-nitridelayer comprises at least one selected from a group consisting of GaN,AlGaN, AN, AlInGaN, InN, and AlInN.
 16. The micropipe-free SiC wafer ofclaim 6, wherein the SiC wafer comprises opposing first and secondsurfaces, an epitaxial layer on at least the first surface of the SiCwafer and a concentration of dopant atoms defining a conductivity forthe epitaxial layer; and a semiconductor device comprising source/drainregions in the epitaxial layer and defining a channel region in theepitaxial layer.
 17. The micropipe-free SiC wafer of claim 16, furthercomprising: a gate dielectric layer formed on the channel region; and ametal gate structure formed on the gate dielectric layer over thechannel region.
 18. The micropipe-free SiC wafer of claim 16, whereinthe semiconductor device comprises at least one of a junctionfield-effect transistor and a hetero-field effect transistor.
 19. Themicropipe-free SiC wafer of claim 6, wherein the SiC wafer comprisesopposing first and second surfaces, an epitaxial layer formed on atleast the first surface, and a concentration of dopant atoms defining aconductivity for the epitaxial layer; and a semiconductor device formedat least in part in the epitaxial layer.
 20. The micropipe-free SiCwafer of claim 19, wherein the semiconductor device comprises at leastone of a Schottky barrier diode, a junction barrier Schottky diode, athyristor, a bipolar junction transistor, and a PiN diode.
 21. Themicropipe-free SiC wafer of claim 6, wherein the SiC wafer comprisesopposing first and second surfaces, an epitaxial layer on at least thefirst surface of the SiC wafer and a concentration of dopant atomsdefining a conductivity for the epitaxial layer; and a semiconductordevice comprising source/drain regions in the epitaxial layer anddefining a channel region in the epitaxial layer.
 22. The micropipe-freeSiC wafer of claim 21, further comprising: a gate dielectric layerformed on the channel region; and a metal gate structure formed on thegate dielectric layer over the channel region.
 23. The micropipe-freeSiC wafer of claim 21, wherein the semiconductor device comprises atleast one of a junction field-effect transistor and a hetero-fieldeffect transistor.
 24. The micropipe-free SiC wafer of claim 6, whereinthe SiC wafer comprises opposing first and second surfaces, an epitaxiallayer formed on at least the first surface, and a concentration ofdopant atoms defining a conductivity for the epitaxial layer; and asemiconductor device formed at least in part in the epitaxial layer. 25.The micropipe-free SiC wafer of claim 24, wherein the semiconductordevice comprises at least one of a Schottky barrier diode, a junctionbarrier Schottky diode, a thyristor, a bipolar junction transistor, anda PiN diode.